text: In organic chemistry, hydrocarbons (compounds composed solely of carbon and hydrogen) are divided into two classes: aromatic compounds and aliphatic compounds (; G. aleiphar, fat, oil). Aliphatic compounds can be saturated (in which all the C-C bonds are single requiring the structure to be completed, or saturated, by hydrogen) like hexane, or unsaturated, like hexene and hexyne. Open-chain compounds, whether straight or branched, and which contain no rings of any type, are always aliphatic. Cyclic compounds can be aliphatic if they are not aromatic.